1. Field of the Invention
The present invention relates to an engine starting device, and more particularly, to an engine starting device which is suitable for overcoming a load of compression stroke of the engine and securely starts the engine.
2. Description of the Related Art
A large torque is required to move a piston beyond a top dead center at a compression stroke of an engine. Therefore, if the engine is started from a position where the piston is stayed at crank angle about 90° before the top dead center, the piston can not often move beyond the top dead center because of high load. An output torque which is high enough to overcome the high load region of the compression stroke is required for a starter that is a motor used for the engine starting device.
If the need could be avoided for starting the engine from such a high load region or from a region immediately before the high load region, it would be possible to overcome the compression stroke even with a starter motor having relatively small torque. Japanese Patent Application Laid-Open No. H7-71350 discloses a starting device in which a crank angle is confirmed at the time of starting of the engine, preliminary rotation including reverse rotation of a predetermined rotation angle or predetermined time corresponding to the crank angle is required and then, normal forward rotation is required. This reference publication also discloses a starting device in which a load torque reducing direction is judged from the crank angle, preliminary rotation is required in the torque reducing direction and then, normal forward rotation is required.
This starting device is realized based on the phenomenon that a friction surface is brought into a substantially dynamical friction surface due to spread of oil caused by reverse rotation, that is preliminary rotation, the friction coefficient is lowered and the load torque is reduced. Enhancement of starting performance is expected as compared with a case in which the engine is normally rotated immediately after the starting command.
In the above conventional starting device, the enhancement of starting performance can be expected to some extent even if a starter motor having not so great starting torque is used. However, this starting device is not sufficient for overcoming the high load region of the compression stroke.
Further, in order to confirm the crank angle as a starting position and to preliminarily rotate reversely only through a rotation angle or for a time period corresponding to the crank angle, detecting means of the starting position is essentially required, and this is not preferable for utilization for general starting devices. Especially when a brushless motor having no position detecting sensor of a rotor is used as a starter motor, it is necessary to provide engine position detecting means as described in Japanese Patent Application Laid-Open No. H7-71350.
In Japanese Patent Application Laid-Open No. H7-71350, if the forward rotation direction is the load torque reducing direction when the crank angle is confirmed, the motor is allowed to rotate forwardly as it is. It is judged whether the starting is successful or failed depending upon whether the engine revolution number exceeds a threshold value revolution number after a predetermined time is elapsed. However, since long time is required for judging whether it is possible to overcome the high load region at the time of forward rotation, there is a problem that too much time is required for restarting when the starting has failed.